26
denition 4
integer 3
set 25, 32
type (of a variable) 17
16.3 Denition:
If x is a member of the set A, one writes x A; if it is not a member
of A, x A.
/
16.3.1 Example 4 Z, 5 Z, but 4/3 Z.
/
16.4
46
denition 4
empty set 33
fact 1
include 43
powerset 46
rule of inference 24
setbuilder notation 27
set 25, 32
subset 43
32. The powerset of a set
32.1 Denition: powerset
If A is any set, the set of
47
33. Union and intersection
33.1 Denition: union
For any sets A and B , the union A B of A and B is dened by
A B = cfw_x | xA xB
(33.1)
33.2 Denition: intersection
For any sets A and B , intersectio
50
coordinate 49
denition 4
integer 3
ordered pair 49
ordered triple 50
specication 2
tuple 50, 139, 140
union 47
usage 2
35.1.2 Method
To prove two ordered pairs x, y and x , y
x = x and y = y .
are
51
36.2.3 Example 1, 3, 3, 2 is a tuple of integers. It has length 4. The integer coordinate 49
empty set 33
3 occurs as an entry in this 4-tuple twice, for i = 2 and i = 3.
36.2.4 Usage Tuples and th
53
37.5 Denition: Cartesian product
Let A1 , A2 , . . . , An be sets in other words, let Ai
of sets. Then A1 A2 An is the set
a1 , a2 , . . . , an | (i:n)(ai Ai )
in
be an n-tuple
(37.1)
of all n-tupl
52
Cartesian product 52 37. Cartesian Products
coordinate 49
denition 4
37.1 Denition: Cartesian product of two sets
diagonal 52
LetA and B be sets. A B is the set of all ordered pairs whose rst
facto
54
Cartesian powers 54
Cartesian product 52
Cartesian square 54
implication 35, 36
include 43
powerset 46
set 25, 32
singleton 34
tuple 50, 139, 140
union 47
37.7.1 For all sets A, B and C , A C = B C
57
39.2.1 Warning This specication for function is both complicated and subtle
and has conceptual traps. One of the complications is that the concept of function
given here carries more information wi
58
codomain 56
divisor 5
domain 56
nite 173
function 56
powerset 46
prime 10
set 25, 32
39.3.4 Example Let S be some set of English words, for example the set of
words in a given dictionary. Then the
59
a) All built-in Mathematica functions, such as Sin, start with a capital letter.
It is customary for the user to use lowercase names so as to avoid overwriting
the Mathematica denition of some func
60
codomain 56
domain 56
equivalence 40
function 56
39.7.1 Method
To show that two functions are the same you have to show they have the
same domain, the same codomain and for each element of the doma
45
31.4.3 Remark The fact that A A for any set A means that any set is a subset denition 4
of itself. This may not be what you expected the word subset to mean. This leads include 43
nontrivial subset
44
denition 4
equivalent 40
hypothesis 36
implication 35, 36
include 43
proof 4
properly included 44
set 25, 32
vacuous 37
31.2 Theorem
a) For any set A, A A.
b) For any set A, A.
c) For any sets A an
43
30.4.3 Example Lets look again at this (true) statement (see Section 10, contrapositive 42
converse 42
page 14):
If the decimal expansion of a real number r has all 0s after a certain
point, it is
22
denition 4
even 5
fact 1
integer 3
negation 22
or 21, 22
positive integer 3
predicate 16
truth table 22
usage 2
14.3 Truth tables
The denitions of the symbols and can be summarized in truth tables:
27
17.1.4 Exercise How many elements does the set cfw_1, 1, 2, 2, 3, 1 have? (Answer comprehension 27,
29
on page 243.)
17.2 Sets in Mathematica
In Mathematica, an expression such as
cfw_2,2,5,6
denot
29
18.1.11 Method: Comprehension
Let P (x) be a predicate and let A = cfw_x | P (x). Then if you know that
a A, it is correct to conclude that P (a). Moreover, if P (a), then you
know that a A.
18.1.1
31
19.2.8 Exercise How many elements does the set
cfw_
1
1 1
| x = , , 2, 2
2
x
2 2
have?
19.3 More about sets in Mathematica
The Table notation described in 17.2 can use the variations described in 1
30
and 21, 22
integer 3
predicate 16
rational 11
real number 12
set 25, 32
unit interval 29
19.1.1 Example The unit interval I could be dened as
I = cfw_x R | 0 x 1
making it clear that it is a set of
32
real number 12
setbuilder notation 27
set 25, 32
specication 2
20.2 Bound and free variables
The variable in setbuilder notation, such as the x in Equation (18.3), is bound, in
the sense that you c
33
21.2.2 Example For x real,
cfw_x | x2 = 1 = cfw_x | (x = 1) (x = 1)
We will prove this using Method 21.2.1. Let
A = cfw_x | x2 = 1 and B = cfw_x | (x = 1) (x = 1)
Suppose x A. Then x2 = 1 by 18.2.
35
24. Russells Paradox
The setbuilder notation has a bug: for some predicates P (x), the notation
cfw_x | P (x) does not dene a set. An example is the predicate x is a set. In
that case, if cfw_x | x
37
26. Vacuous truth
The last two lines of the truth table for implication mean that if the hypothesis of
an implication is false, the implication is automatically true.
26.1 Denition: vacuously true
42
30. Statements related to an implication
contrapositive 42
converse 42
decimal expansion 12
30.1 Denition: converse
decimal 12, 93
The converse of an implication P Q is Q P .
denition 4
equivalent
63
40.3 Explicit denitions of function
In many texts, the concept of function is dened explicitly (as opposed to being
given a specication) by some such denition as this: A function F is an ordered
tr
61
40. The graph of a function
40.1 Denition: graph of a function
The graph of a function F : A B is the set
a, F (a) | a A
of ordered pairs whose rst coordinates are all the elements of A with
the se